The present invention relates to a passive infrared detector having a heat-sensitive sensor and a focusing device for focusing thermal rays incident from the room under surveillance incident on the detector on to the sensor, and more particularly the focusing device having focusing elements for the surveillance regions having different positions in the room under surveillance.
Passive infrared detectors of this type have been known for years and are widespread. They serve, in particular, to detect the presence of unauthorized individuals into the room under surveillance by detecting the typical infrared radiation that is emitted by individuals which is guided by a focusing element onto the sensor. Known focusing devices include Fresnel lenses that are incorporated into the entrance window for the infrared radiation disposed on the front of the detector casing (in this connection, see, for example, EP--A-0 559 110) or a mirror that is disposed in the interior of the detector casing and that comprises individual reflectors (in this connection, see U.S. Pat. No. 4,880,980). Generally, a plurality of rows of reflectors is provided, each row corresponding to a particular surveillance zone, for example, remote zone, middle zone, near zone and look-down zone.
Both the Fresnel lenses and the mirrors are designed so that each surveillance zone is divided into surveillance regions and the room to be kept under surveillance is thus covered in a fanshaped manner by surveillance regions emanating from the detector. Consequently, each reflector determines a surveillance region with a defined position in the room under surveillance. As soon as an object emitting thermal radiation intrudes into the room, the sensor detects the thermal radiation emitted by the object. The detection is most reliable if the object moves transversely with respect to the surveillance region.
Although passive infrared detectors of the present generation can detect intruders within the active region of the detector very reliably, they are not generally able to distinguish human beings from fairly large domestic animals, such as, for example, dogs, and emit an alarm even when an animal is detected. The longer these false alarms are, the less they are tolerated and the protection of passive infrared detectors, against false alarms triggered by domestic animals moving through the room under surveillance, described as domestic animal immunity, has recently developed as an essential requirement of the market. This feature is increasingly being demanded even of passive infrared detectors in the lower price segment of the market.
Those passive infrared detectors that already have domestic animal immunity at present generally achieve this feature by reducing the response sensitivity of the detector, which results in an undesirable reduction in the detection reliability.
In a passive infrared detector having domestic animal immunity described in U.S. Pat. No. 4,849,635, the focusing device is formed by a lens arrangement having a plurality of differently aligned, non-overlapping fields of view or surveillance regions that extend in a fan-shaped manner from the lens arrangement into the room under surveillance. These surveillance regions are staggered vertically, approximately equally such that relatively large gaps are formed between the individual regions. An intruder having a certain minimum height will always cross at least one surveillance region and consequently always generate a sensor signal. An intruder below the minimum height will cross surveillance regions and gaps only alternately and in the latter case will not generate a sensor signal. In this way, a human being, if he moves through the room under surveillance will generate a steady sensor signal having approximately constant amplitude, whereas an animal triggers a pulse-shaped signal of substantially lower maximum amplitude.
Since, in this known system, human beings and domestic animals are distinguished on the basis of the signal shape and since the vertical staggering of the surveillance regions is an equipment constant, there is a relatively great danger that large domestic animals cannot be distinguished from small human beings and vice versa.
The object of the invention is therefore to provide a passive infrared detector of the type mentioned at the outset whose ability to distinguish between human beings and animals is substantially improved.
The object is achieved, according to the invention, in that each focusing element comprises a number of sub-elements so that the surveillance regions are split up vertically into subzones having slightly different elevation and in that the human beings are distinguished from animals on the basis of the amplitude of the sensor signal.
The achievement according to the invention has the advantage that even a very large animal is always reliably distinguished from a human being provided its height is less than that of an human being. After all, a human being walking upright still always crosses a plurality of subzones of remote and middle zones, or middle and near zones, etc., and therefore triggers a much greater sensor signal than an animal of smaller height. The latter will cross markedly fewer subzones and generate a markedly reduced sensor signal. A dog of normal height will cross one subzone or at most two, but this only partly, and will consequently trigger a signal reduced to one half or one third compared with the detector described in U.S. Pat. No. 4,880,980.
A first embodiment of the passive infrared detector according to the invention is characterized in that the elevation of the sub-elements is chosen so that, in the majority of the surveillance regions, at most only an insignificant overlapping of the subzones occurs.
A second embodiment is characterized in that the number of sub-elements and, correspondingly, the number of subzones increases with decreasing radial distance of the respective surveillance region from the detector.
A third embodiment of the detector according to the invention is characterized in that the subzones are arranged in layers in a stack-like manner on top of one another and that the chosen layering is such that a sequence of dense curtains is produced and the sensitivity in the individual subzones being approximately equal. The latter is achieved by avoiding overlapping of the individual subzones.
A fourth embodiment of the detector according to the invention is characterized in that the weighting of the individual sub-elements, in particular their optical aperture and area, is chosen in such a way that an animal that is moving transversely with respect to the coverage pattern formed by the surveillance region and that is of any optional size delivers an approximately equally small signal for all distances between animal and detector. Preferably, the animal is a hair-coated dog with a length of 80 cm and a height of 60 cm.
A fifth embodiment of the detector according to the invention is characterized in that the focusing device is formed by a mirror arrangement having reflectors forming the focusing elements and each reflector is split up into sub-areas. The sub-areas, which are, as a rule, paraboloid sub-areas, can be combined to form groups of mirror regions that are joined together for the production of the injection-molding tool for the mirror arrangement, resulting in a less expensive production and maintenance of the injection-molding tool.
A sixth embodiment is characterized in that the mirror arrangement has a first reflector row for a remote zone, a second reflector row for a middle zone, a third reflector zone for a near zone and a fourth reflector row for a look-down zone and in that the reflectors of the first row and the reflectors of the second row are each split up into three sub-areas and the reflectors of the third row are split up into four sub-areas and the reflector of the fourth row is split up into five sub-areas.
A further embodiment of the detector according to the invention is characterized in that the sensor has four sensor elements that are combined in pairs and that form two independent channels and in that the respective signal is evaluated in each channel.